The present invention relates to curable epoxy-resin-containing formulations, and particularly to formulations useful for making laminates for printed wiring boards.
It is known to make electrical laminates and other composites from a fibrous reinforcement and an epoxy-containing matrix resin. Examples of suitable processes usually contain the following steps:
(1) an epoxy-containing formulation is applied to a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers.
(2) The impregnated substrate is xe2x80x9cB-stagedxe2x80x9d by heating at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, so that the impregnated substrate can be handled easily. The xe2x80x9cB-stagingxe2x80x9d step is usually carried out at a temperature of from 90xc2x0 C. to 210xc2x0 C. and for a time of from 1 minute to 15 minutes. The impregnated substrate that results from B-staging is called a prepreg. The temperature is most commonly 100xc2x0 C. for composites and 130xc2x0 C. to 200xc2x0 C. for electrical laminates.
(3) One or more sheets of prepreg are stacked in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired.
(4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of lamination is usually between 100xc2x0 C. and 230xc2x0 C., and is most often between 165xc2x0 C. and 190xc2x0 C. The lamination step may also be carried out in two or more stages, such as a first stage between 100xc2x0 C. and 150xc2x0 C. and a second stage at between 165xc2x0 C. and 190xc2x0 C. The pressure is usually between 50 N/cm2 and 500 N/cm2. The lamination step is usually carried on for from 1 to 200 minutes, and most often for 45 to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes).
(5) Optionally, the resulting copper-clad laminate may be post-treated by heating for a time at high temperature and ambient pressure. The temperatures of post-treatment are usually between 120xc2x0 C. and 250xc2x0 C. The post-treatment time usually is between 30 minutes and 12 hours.
It is conventional in the preparation of epoxy-containing laminates to incorporate into the epoxy resin composition various additives to improve the flame-retardancy of the resulting laminate. Many types of flame retardant additives have been suggested, but the additives which are most widely used commercially are halogen containing additives, such as tetrabromodiphenylolpropane, or epoxy resins prepared by reacting diglycidyl ether of bisphenol-A with tetrabromodiphenylolpropane. Typically, in order to reach the desired fire retardancy level (V-0 in the standard xe2x80x9cUnderwriters Laboratoryxe2x80x9d test method UL 94) levels of such compounds are required which provide a bromine content of from 10 to 25 weight percent based on the total polymer weight in the product.
Although halogen containing fire-retardant additives are effective, they are considered by some to be undesirable from an environmental standpoint, and in recent years there has been increasing interest in the formulation of halogen-free epoxy resins, which are able to meet the fire retardancy requirements.
Proposals have been made to use phosphorus based flame retardants instead of halogenated fire retardants (see, for example EP-A-0384939, EP-A-0384940, EP-A-0408990, DE-A-4308184, DE-A-4308185, DE-A-4308187, WO-A-96/07685, and WO-A-96/07686. In these formulations the phosphorus flame retardant was pre-reacted to form a di or multifunctional epoxy resin. This makes the epoxy resin relatively expensive.
The phosphonic acid esters are commercially available fire retardant materials (e.g., Amgard(trademark) V19 and Amgard(trademark) P45xe2x80x94supplied by Albright and Wilson Ltd, United Kingdom). These phosphonic acid esters, may be solids or liquids. Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins. In particular lower (i.e., C1-C4) alkyl esters of phosphonic acid are of value because they contain a high proportion of phosphorus, and are thus able to impart good fire retardant properties upon resins in which they are incorporated. However, it has been found by the present inventors that they are not satisfactory as a substitute for halogenated flame retardants in epoxy resins for the production of electrical laminates, because their use in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of the cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260xc2x0 C., a typical step in the manufacture of printed wiring boards.
EP-A-0754728 describes the production of flame retardant epoxy resin systems by blending epoxy resins with phosphonic acid esters and incorporating them into the cured resin. This reference indicates that large quantities (in excess of 18 weight percent) of the phosphorus additive are needed in order for the resin system to meet UL 94 V-0.
We have now discovered that it is possible to produce epoxy resins which meet the desirable standards of fire retardancy without the need for halogen-containing flame retardants, or at least employing significantly lower levels of such halogen-containing flame retardants than is conventional in the art, by employing relatively low levels of phosphonic acid ester flame retardant (to give 0.2 to 5 weight percent phosphorus in the solid resin), together with particular combinations of accelerator and catalyst, and, in preferred embodiments, particular types of epoxy resin. The accelerators and catalysts are generally known per se, but their use in conjunction with low levels of phosphonic acid ester fire retardants to obtain compositions which have both good fire retardancy, and yet sufficiently low water absorption has not hitherto been described.
According to the invention, there is provided a flame retardant epoxy resin composition containing no more than 10% by weight of halogen, comprising
a) an epoxy resin,
b) a phosphonic acid ester in an amount such as to provide from 0.2 to 5 weight percent phosphorus in the composition,
c) a nitrogen-containing crosslinking agent having an amine functionally of at least 2, in an amount of from 10 to 80 percent of the stoichiometric amount needed to cure the epoxy resin,
d) from 0.1 to 3 weight percent of a catalyst capable of promoting the reaction of the phosphonic acid ester with the epoxy resin and promoting the curing of the epoxy resin with the crosslinker, and optionally,
e) a Lewis acid in an amount of up to 2 moles, per mole of catalyst.
The phosphonic acid ester employed in the present invention is preferably an ester of the formula 
wherein R1 is C1 to C3 alkyl,
R4 is C1 to C3 alkylene,
R2, and R3, are each C1 to C3 alkyl, or C6 to C10 aryl, or R2 and R3 taken together represent the residue of a glycol or a polyol.
Preferred phosphonic acid esters are for example esters of methanephosphonic acid with polyhydroxy compounds such as glycols, and polyols. Such phosphonic acid esters of polyhydroxy compounds can have polymeric and/or cyclic structures.
Specific preferred examples are polymers with repeating units such as: 
and/or cyclic structures such as 
n is 2 to 10, R5 is a C1 to C3 alkylene group or the residue of a glycol or polyol, 
R6 is the residue of a triol, for example glycerol or trimethylol propane.
The phosphonic acid ester is preferably an ester having methyl or methylene adjacent to phosphorous. Preferred phosphonic acid esters are those of the formula, 
In order to obtain satisfactory resistance to water absorption, it is important that the amount of the phosphonic acid ester is from 1 to 18 weight percent, preferably from 4 to 15 weight percent, more preferably from 7 to 15 weight percent, based on the total of the epoxy resin composition. The phosphonic acid ester content is preferably such as to provide a total phosphorous content in the composition, of from 0.2 to 5 weight percent, more preferably from 1 to 5 weight percent.
The compositions of the invention contain a catalyst capable of promoting the reaction of the phosphonic acid ester with the epoxy resin and promoting the curing of the epoxy resin.
The catalyst may comprise a single catalyst component, capable both of promoting the reaction of the phosphonic acid ester with the epoxy resin and of promoting the curing of the epoxy resin. Alternatively,the catalyst may be a combination of components which favor one aspect of the resin curing over another.
Examples of suitable catalyst materials include compounds containing or amine, phosphine, ammonium, phosphonium, arsonium or sulfonium moieties. Particularly preferred catalysts are the heterocyclic nitrogen containing compounds. Preferably a Lewis acid is also employed, especially when the catalyst is an amine, particularly a heterocyclic nitrogen containing amine. Catalysts (as distinguished from cross-linkers) preferably contain on average no more than about 1 active hydrogen moiety per molecule. Active hydrogen moieties include hydrogen atoms bonded to an amine group, a phenolic hydroxyl group, or a carboxylic acid group. For instance, the amine and phosphine moieties in catalysts are preferably tertiary amine or phosphine moieties; and the ammonium and phosphonium moieties are preferably quaternary ammonium and phosphonium moieties.
Among preferred tertiary amines that may be used as catalysts are those mono- or polyamines having an open-chain or cyclic structure which have all of the amine hydrogen replaced by suitable substituents, such as hydrocarbyl radicals, and preferably aliphatic, cycloaliphatic or aromatic radicals.
Examples of these amines include, among others, methyl diethanol amine, triethylamine, tributylamine, dimethyl benzylamine, triphenylamine, tricyclohexyl amine, pyridine and quinoline. Preferred amines are the trialkyl, tricycloalkyl and triaryl amines, such as triethylamine, triphenylamine, tri-(2,3-dimethylcyclohexyl)amine, and the alkyl dialkanol amines, such as methyl diethanol amines and the trialkanolamines such as triethanolamine. Weak tertiary amines, for example, amines that in aqueous solutions give a pH less than 10 in aqueous solutions of 1 M concentration, are particularly preferred. Especially preferred tertiary amine catalysts are benzyldimethylamine and tris-(dimethylaminomethyl) phenol.
Examples of suitable heterocyclic nitrogen catalysts include those described in U.S. Pat. No. 4,925,901. Preferable heterocyclic secondary and tertiary amines or nitrogen-containing catalysts which can be employed herein include, for example, imidazoles, benzimidazoles, imidazolidines, imidazolines, oxazoles, pyrroles, thiazoles, pyridines, pyrazines, morpholines, pyridazines, pyrimidines, pyrrolidines, pyrazoles, quinoxalines, quinazolines, phthalozines, quinolines, purines, indazoles, indoles, indolazines, phenazines, phenarsazines, phenothiazines, pyrrolines, indolines, piperidines, piperazines and combinations thereof. Especially preferred are the alkyl-substituted imidazoles; 2,5-chloro-4-ethyl imidazole; and phenyl-substituted imidazoles, and mixtures thereof. Even more preferred are N-methylimidazole; 2-methylimidazole; 2-ethyl-4-methylimidazole; 1,2-dimethylimidazole; and 2-methylimidazole. Especially preferred is 2-phenylimidazole.
Examples of heterocyclic nitrogen donor compounds which are preferably used in combination with Lewis acids are those described in EP-A-526488, EP-A-0458502- and GB-A-9421405.3. In these references the Lewis acids are described as inhibitors because they reduce the initial rate of the chemical reaction. Examples of suitable Lewis acids include halides, oxides, hydroxides and alkoxides of zinc, tin, titanium, cobalt, manganese, iron, silicon, aluminium, and boron, for example Lewis acids of boron, and anhydrides of Lewis acids of boron, for example boric acid, metaboric acid, optionally substituted boroxines (such as trimethoxyboroxine), optionally substituted oxides of boron, alkyl borates, boron halides, zinc halides (such as zinc chloride) and other Lewis acids that tend to have a relatively weak conjugate base. Preferably the Lewis acid is a Lewis acid of boron, or an anhydride of a Lewis acid of boron, for example boric acid, metaboric acid, an optionally substituted boroxine (such as trimethoxy boroxine, trimethyl boroxine or triethyl boroxine), an optionally substituted oxide of boron, or an alkyl borate. The most preferred Lewis acid is boric acid.
These Lewis acids are very effective in curing epoxy resins when combined with the heterocyclic nitrogen containing compounds, referred to above. In particular, they are able to incorporate the phosphonic acid ester into the epoxy resin during curing.
The Lewis acids and amines can be combined before mixing into the formulation or by mixing with the catalyst in-situ, to make a curing catalyst combination.
The amount of the Lewis acid employed is preferably at least 0.1 moles of Lewis acid per mole of heterocyclic nitrogen compound, more preferably at least 0.3 moles of Lewis acid per mole of heterocyclic nitrogen compound.
The formulation preferably contains no more than three moles of Lewis acid per mole of catalyst and more preferably contains no more that 2 moles of Lewis acid per mole of catalyst. The total amount of the catalyst is from 0.1 to 3 weight percent, based on the total weight of the composition, preferably from 0.1 to 2 percent.
All of the aforementioned catalyst materials will catalyse to some extent both the reaction of the phosphonic acid ester with the epoxy resin, and the curing of the epoxy resin. However, (1,8-diazabicyclo (5,4,0) undec-7-en (DBU) tends to favor the phosphonic ester-epoxy reaction over the epoxy curing reaction.
The nitrogen-containing crosslinking agent has an amine functionality of at least 2. Suitable multifunctional crosslinkers are described in numerous references such as Vol. 6 Encyclopaedia of Poly. Sci. and Eng., xe2x80x9cEpoxy resinsxe2x80x9d at 348-56 (J. Wiley and Sons 1986). Examples of suitable nitrogen-containing cross-linkers include polyamines, polyamides, sulphanilamide, diaminodiphenylsulfone and diaminodiphenyl methane. The preferred crosslinking agent is dicyandiamide.
The amount of the nitrogen-containing crosslinking agent is from 10 to 80 percent of the stoichiometric quantity needed to cure the epoxy content of the epoxy resin in the formulation.
The total amount of nitrogen in the composition (including nitrogen derived from any nitrogen-containing compounds which may form part of the catalyst, or advancement monomers for the epoxy resin) is preferably from 1 to 8 percent by weight.
The quantity of nitrogen-containing cross-linking agent is preferably such that the formulation contains a stoichiometric excess of epoxy resin over the nitrogen-containing cross-linking agent. (For the purposes of this application, dicyandiamide is taken as having 6 curing sites per molecule.) The formulation thus contains no more than 0.8 equivalents, preferably no more than 0.75, more preferably no more than 0.6, and most preferably no more than 0.5 equivalents of nitrogen-containing cross-linking agent per epoxide equivalent. When the multifunctional cross-linker is dicyandiamide, the formulation preferably contains at least 0.65 weight percent and more preferably at least 1.9 weight percent of dicyandiamide. The amount of dicyandiamide is preferably no more than 5.2 weight percent and more preferably no more than 2.6 weight percent.
The epoxy resin used in the present invention is a material which possesses on average more than 1 and preferably at least 1.8, more preferably at least 2 epoxy groups per molecule. In the broadest aspect of the invention, the epoxy resin may be any saturated or unsaturated aliphatic, cycloaliphatic, aromatic or heterocyclic compound which possesses more than one 1,2-epoxy group. Examples of heterocyclic epoxy compounds are diglycidylhydantoin or triglycidyl isocyanurate (TGIC).
The epoxy resin is preferably one which has no lower alkyl aliphatic substituents, for example the glycidyl ether of a phenol novolac, or the glycidyl ether of bisphenol-F.
The most preferred epoxy resins are epoxy novolac resins (sometimes referred to as epoxidised novolac resins, a term which is intended to embrace both epoxy phenol novolac resins and epoxy cresol novolac resins). Such compounds have the following general formula: 
wherein xe2x80x9cRxe2x80x9d is hydrogen or a C1-C3 alkyl, e.g., methyl and xe2x80x9cnxe2x80x9d is 0 or an integer from 1 to 10.
Epoxy novolac resins (including epoxy cresol novolac resins) are readily commercially available, for example under the trade names D.E.N.(trademark), Quatrex(trademark), Tactix(trademark), (Trademarks of The Dow Chemical Company). The materials of commerce generally comprise mixtures of various species of the above formula and a convenient way of characterizing such mixtures is by reference to the average, nxe2x80x2, of the values of n for the various species. Preferred epoxy novolac resins for use in accordance with the present invention are those in which nxe2x80x2, has a value of from about 2.05 to about 10, more preferably from about 2.5 to about 5.
Preferably the epoxy resin is the reaction product of an epoxy compound containing at least two epoxy groups for example an epoxy compound of the kind describe above, and a chain extender. The chain extending monomer may be a phenolic chain extender containing on average more than one, and less than three phenolic hydroxyl groups per molecule. Such phenolic chain extenders preferably contain on average 1.8 to 2.1 phenolic hydroxyl groups and more preferably contains about 2 phenolic hydroxyl groups per molecule. The phenolic chain extender is preferably a dihydric phenol. The chain extender is preferably reacted with the epoxy compound to form the epoxy resin before formulation of the composition with the flame retardant, hardener and the catalyst. However it is also possible to add the chain extender and the epoxy resin compound to the composition in order to form the epoxy resin in-situ.
The epoxy resin is preferably one which is solid at 20 degrees C., for example one which has a softening point of 50xc2x0 C. or higher, as defined by the Mettler softening point method ASTM D3104 and DIN 51920. The phenolic chain extender may itself be the reaction product of a diol and an epoxy compound.
For example, it may be the reaction product of a diol or a compound containing two phenolic groups, with a glycidyl ether of a phenol novolac or with a glycidyl ether of bisphenol-F. Preferably, less than 50 percent of the carbon atoms in the chain extender are present in aliphatic groups, more preferably less than 30 percent, and most preferably 0 percent.
Examples of particularly suitable phenolic chains extenders include resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenylethane), bisphenol F, and bisphenol K.
It is greatly preferred however that the chain extender is a nitrogen-containing monomer for example, an isocyanate, and amine or amide.
Preferred nitrogen-containing chain extenders include, polyisocyanate compounds which form epoxy-terminated polyoxazolidones as described in U.S. Pat. No. 5,112,932. Preferably, the polyisocyanate compound used in the present invention is methylene bis(phenylisocyanate) (MDI). MDI is preferably employed in its commercially available form, which includes, pure 4-4, MDI, isomers and functional homolog mixtures (commonly designated as xe2x80x9cpolymeric MDIxe2x80x9d). Isocyanate compounds also useful in the present invention include, for example, toluene diisocyanate (TDI) and isomers thereof.
The nitrogen-containing chain extender may also be, for example an amine- or amino amide-containing compound which forms epoxy-terminated amine compounds having two Nxe2x80x94H bonds capable of reacting with an epoxy group. Amine-containing compounds useful in the present invention include, for example, mono-primary amines of the general formula R-NH2 wherein R is alkyl, cycloalkyl or aryl moieties; di-secondary amines of the general formula Rxe2x80x94NHxe2x80x94Rxe2x80x2xe2x80x94NHxe2x80x94Rxe2x80x3 wherein R, Rxe2x80x2 and Rxe2x80x3 are alkyl, cycloalkyl or aryl moieties; and heterocyclic di-secondary amines wherein one or both of the N atoms is part of a nitrogen containing heterocyclic compound such as: 
For reactivity reasons, and in order to better control the epoxy advancement reaction with the di-functional amines, di-secondary amines or primary amines having sterically hindered amine groups are preferred as for example 2,6-dimethyl cyclohexylamine or 2,6-xylidine (1-amino-2,6-dimethylbenzene).
Amino amide-containing compounds useful as chain extenders in the present invention include for example derivatives of carboxylic acid amides as well as derivatives of sulfonic acid amides having additionally one primary or two secondary amino groups. Preferred examples of such compounds are amino-aryl carboxylic acid amides and amino-arylsulfonamides. A preferred compound of this group is, for example, sulfanilamide (4-amino benzenesulfonamide) and anthranilamide(2-aminobenzamide).
The amount of the chain extender is preferably from 5 to 30 weight percent, based on the epoxy resin.
The compositions of the invention may also contain one or more additional flame retardant additives, for example red phosphorous or liquid or solid phosphorus containing compounds, for example, ammonium polyphosphate, a phosphite, or 9,10-dihydro-9-oxa-phosphaphenanthrene-10-oxide (HCA), phosphazenes, nitrogen containing fire retardants and/or synergists, for example melamines, urea, cynamide, guanidine, cyanuric acid, isocyanuric acid and derivatives of those nitrogen containing compounds, halogenated flame retardants, halogenated epoxy resins (especially brominated epoxy resins) synergistic phosphorus-halogen containing chemicals or compounds containing salts of organic acids, inorganic metal hydrates, boron or antimony. Examples of suitable additional flame retardant additives are given in a paper presented at xe2x80x9cFlame retardantsxe2x80x94101 Basic Dynamicsxe2x80x94Past efforts create future opportunitiesxe2x80x9d, Fire Retardants Chemicals Association, Baltimore Marriot Inner Harbour Hotel, Baltimore Md., Mar. 24-27 1996. When additional fire retardants are present which contain phosphorous, they are generally present in amounts such that the total phosphorous content of the epoxy resin composition is from 0.2 to 5 weight percent.
The compositions of the invention can be produced by mixing all the components together in any order. Preferably compositions of the invention can be produced by preparing a first composition comprising the epoxy resin, and the second composition comprising the curing catalyst. Either the first or the second composition also comprises the phosphonic acid ester, nitrogen-containing crosslinking agents. All other components may be present in the same composition, or some may be present in the first, and some in the second. The first composition is then mixed with the second composition, and cured to produce a fire retardant epoxy resin.